NOTICE: this is the author’s version of a work that was accepted for publication in Nuclear Instruments & Methods in Physics Research Section B Beam Interactions with Materials and Atoms. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Nuclear Instruments & Methods in Physics Research Section B Beam Interactions with Materials and Atoms, 2011, v. 269 n. 13, p. 1523-1526. DOI: 10.1016/j.nimb.2011.04.107